Some conventional intake manifolds include a gas introduction section for introducing into a surge tank a gas containing mist-like oil component or water vapor such as blowby gas in a crankcase, a PCV gas from a canister, an EGR gas (exhaust gas recirculation gas), etc. and include also a negative pressure feed passage for feeding an intake negative pressure inside the surge tank to the outside (e.g. a brake booster, etc.). With such intake manifolds, there is a concern that fuel component and water contained in the gas introduced from the gas introduction section may inadvertently enter the negative pressure feed passage and freeze therein to block a negative pressure feed port.
As an intake manifold for solving the above concern, there is disclosed a technique wherein a concentration port is provided to communicate with a main flow passage for air and the concentration port includes a gas introduction section (referred to as “a gas introduction port” in the document) for introducing a gas containing mist-like fluid or vapor and a negative pressure feed passage (referred to as “a negative pressure introduction port” in the document) for introducing a negative pressure, and a partition wall portion is provided between the gas introduction section and an opening of the negative pressure feed passage (see Patent Document 1).
Also disclosed is a technique wherein there is provided a raised portion at an appropriate position in an inner face of a wall portion constituting the surge tank, the raised portion being raised therefrom and forming a step relative thereto and there is provided also a gas introduction section (referred to as “a gas introduction hole” in the document) provided at an appropriate position of the wall portion inner face other than the raised portion for introducing a water vapor containing gas, a negative pressure feed passage (referred to as “an intake negative pressure outlet hole” in the document) for taking the intake negative pressure inside the surge tank to the outside is connected to the raised portion and in the raised face of the raised portion and at an area thereof located upwardly of an opening of the negative pressure feed passage, there is provided a guide groove for receiving water dropped along the wall portion inner face upwardly of the raised portion and guiding the received water to a position away from the negative pressure feed passage (see e.g. Patent Document 2).